MANCHESTER, England (Reuters) - British Prime Minister David Cameron said on Sunday (Oct 4) that Russian President Vladimir Putin's decision to take military action in Syria to support Syrian President Bashar al-Assad was a "terrible mistake".

"They are backing the butcher Assad, which is a terrible mistake for them and for the world. It's going to make the region more unstable," Mr Cameron told the BBC on the first day of his Conservative Party's annual conference in the northern city of Manchester.

Russia this week struck at targets in Syria, a dramatic escalation of foreign involvement in the civil war.

"Most of the Russian airstrikes, as far as we've been able to see so far, have been in parts of Syria not controlled by ISIL (Islamic State in Iraq and Syria), but controlled by other opponents of the regime," Mr Cameron said.